The photoacoustic spectroscopy is a method in which a pulsed light having a predetermined wavelength (e.g., wavelength range of visible light, near infrared light, or mid-infrared light) is projected onto a subject and a photoacoustic wave, which is an elastic wave generated as a result of absorption of the energy of the pulsed light by a specific substance in the subject, is detected, thereby quantitatively measuring the concentration of the specific substance (e.g., Japanese Unexamined Patent Publication No. 2010-012295). For example, the specific substance in the subject may be glucose, hemoglobin, or the like included in the blood. The technology that detects a photoacoustic wave in the manner described above and generates a photoacoustic image based on the detected signal is called photoacoustic imaging (PAI) or photoacoustic tomography (PAT).
There has conventionally been a problem described hereinbelow in the measurements using the photoacoustic spectroscopy described above (photoacoustic measurements). That is, the intensity of the light projected onto the subject is significantly attenuated due to absorption and scattering in the process of traveling through the subject. The intensity of a photoacoustic wave generated in the subject based on the projected light is also attenuated due to absorption and scattering in the process of propagating through the subject. Consequently, it is difficult to obtain information of a deep portion of the subject by the photoacoustic measurements. In order to solve this problem, it is conceivable that, for example, the magnitude of generated photoacoustic wave is increased by increasing the amount of light energy projected into the subject through the use of high energy light.
In the case where high energy (1 mJ or more) light required in photoacoustic measurements is guided by an optical fiber, it is highly likely that the end face of the optical fiber is damaged and a durability problem of the optical fiber may possibly occur. Generally, when light is inputted to an optical fiber, the end face of the optical fiber is placed adjacent to the focal position of the condenser lens so that the beam diameter of the light fits into the core diameter of the optical fiber. Here, when the light is condensed by the condenser lens, however, the light is focused too narrowly and the energy is locally concentrated, whereby end face damage of the optical fiber may progress from the energy concentrated point as the origin.
In the meantime, for example, Japanese Unexamined Patent Publication No. 2004-193267 discloses that transmission of high energy light is realized by the use of a bundle fiber whose entrance end is fusion processed (fused bundle fiber) to efficiently reduce the light energy incident on a unit area.